1 66 PHYSIOLOGICAL CHEMISTRY. 



a part of the acid combined at once with the albumin and is left in a form 

 not yet soluble. To the third portion (C) of 25 cc. measured oft add 2 

 drops of the weak dimethylaminoazobenzene indicator and titrate directly 

 with the N/io alkali. The result will be found distinctly lower than that 

 with A or B, even in this beginning stage of the process. 



The remainder of the albumin and acid mixture in a loosely stoppered 

 flask is placed in a water-bath and kept as exactly as possible at a tem- 

 perature of 40 C. through five or six hours. The mixture is shaken 

 frequently as in the pepsin test described above. At the end of two or 

 three hours measure out two portions of 25 cc. each; titrate one with 

 phenol-phthalein addition and the other with dimethylaminoazobenzene. 

 The result with phenol-phthalein present should be exactly the same as 

 before, while with the other indicator it will probably be a little less than 

 in the first case and not much more than half the acidity shown by the 

 phenol-phthalein. After the digestion has continued six hours, or until 

 practically complete, test two further portions of 25 cc. each in the same 

 way. The total acidity as measured by the aid of phenol-phthalein will be 

 found constant, while with the dimethylaminoazobenzene the " free " acid 

 should be found still further lowered probably, and not over half the total 

 acid. The exact relation of the free to the total acidity depends on the 

 strength of the pepsin and the amount of albumin taken. In a long- 

 continued artificial digestion or in presence of much pepsin the acid is 

 gradually combined more and more completely because the basic digestion 

 products formed have relatively lower molecular weights and combine 

 with the acid more or less perfectly. Exact numerical relations here have 

 not yet been established by sufficiently numerous or detailed experiments. 



Titration of the Gastric Juice. The illustration given 

 above shows about how this should be carried out. In gen- 

 eral as large a volume as these used will not be available, but 

 5 or 10 cc. should be collected by the tube or otherwise for 

 each test. In testing for the total acidity the mixture should 

 not be filtered, unless the digestion is far advanced, for the 

 reason just pointed out. A part of the hydrochloric acid may 

 be held in the insoluble residue. In testing for the free acid, 

 however, the measured portion should be filtered and the resi- 

 due on the filter washed with a little water. The whole of the 

 free acid will then be found in the filtrate. As the color 

 change with the indicator here used is not as sharp as in the 

 other case a clearer liquid is essential for the test. 



These two titrations give us the total acidity and the free 

 hydrochloric acid, but do not measure the organic acid which 

 may possibly be present. Attempts have been made to esti- 



